MAX phase-derived woolen ball-like K2Ti8O17 with excellent surface-enhanced Raman scattering property

In surface-enhanced Raman scattering (SERS) applications, the MAX phase is typically acid etched to address deficiencies, while the more accessible alkali corrosion products are neglected. The woolen ball-like K₂Ti₈O₁₇ (KTO) nanomaterial is synthesized via an efficient hydrothermal surface corrosion reaction between KOH solution and MAX phase Ti₂AlN, which exhibits excellent SERS capabilities to the contaminating dyes. The enhancement factors are 2.33 × 10⁵, 1.04 × 10⁵ and 2.22 × 10⁵ with lowest limits of detection of 10⁻⁷ M, 10⁻⁶ M and 10⁻⁷ M for crystal violet, rhodamine 6G and methylene blue, respectively, indicating that KTO is a highly desired candidate of SERS substrate material. Meanwhile, KTO shows excellent SERS performance for chrysoidine in simulated seawater, which proves its practical application value. The excellent SERS performance of KTO is attributed to the charge transfer mechanism, which is made possible by the appropriate energy band structure and the robust adsorption capacity. In conclusion, a novel method for the synthesis of KTO is investigated and its reaction process and enhancement mechanism are exhaustively characterized and described. The woolen ball-like KTO exhibits remarkable SERS properties and potential applications.     

A high-performance supercapacitor electrode based on freestanding N-doped Ti3C2Tx film

Two-dimensional transition metal carbide (MXene) is a promising electrode material for supercapacitors because of its excellent electrochemical properties. Here, we report a controllable and facile strategy to prepare a freestanding and flexible N-doped Ti₃C₂T_x (N–Ti₃C₂T_x) film electrode with a hydrothermal method using hydrazine hydrate (N₂H₄∙H₂O) as a nitrogen source. At a scan rate of 2 mV s⁻¹, the N–Ti₃C₂T_x film electrode exhibits a high specific capacitance of 340 F g⁻¹ and no capacitance degradation after 10,000 cycles in 1 M H₂SO₄ electrolyte. These results show that the N–Ti₃C₂T_x film could be used as an outstanding electrode material for high-performance supercapacitors. The operation of hydrazine treatment provides a more practical and convenient experimental method for N-doping.     

Novel synthesis method and microstructure evolution of Ti3C2(OH)2/K2Ti8O17 nanocomposites as an effective surface enhanced Raman scattering substrate

In this paper, we have developed a mild and facile approach using KOH solution as etchants to corrode Ti₃SiC₂ at room temperature, fabricating novel Ti₃C₂(OH)₂/K₂Ti₈O₁₇ nanocomposites with open network structures and abundant surface functional groups. The as-produced products obtain the excellent surface enhanced Raman scattering (SERS) performance in the determination of crystal violet (CV), rhodamine 6G (R6G) and methylene blue (MB) dye molecules. The enhancement factors of the three can be calculated to be 3.98 × 10⁵, 1.78 × 10⁵ and 5.27 × 10⁵, and the Raman signals can be detected at the concentrations as low as 10^?6 M, 10^?8 M and 10^?6 M, respectively, suggesting the potential for reliable SERS substrates. In this regard, this work provides a new strategy for exploring and synthesizing MAX phase-derived nanomaterials as promising SERS substrates for high-sensitive molecular detection.     

Ti3C2Tx aerogel with 1D unidirectional channels for high mass loading supercapacitor electrodes

As one of the typical MXenes materials, 2D Ti₃C₂T_x has attracted extensive attention in the field of energy storage. However, due to the restacking problem of Ti₃C₂T_x nanosheets, the electrochemical performance of Ti₃C₂T_x is unsatisfactory. In this paper, a scheme is proposed to obtain 3D aerogel with 1D channels by directional freeze drying of Ti₃C₂T_x. With the help of the unidirectional channels, the 3D Ti₃C₂T_x/Sodium alginate (SA) aerogel can effectively solve the stacking problem of Ti₃C₂T_x nanosheets, and it also accelerates the diffusion of ions. The Ti₃C₂T_x/SA-5 electrode can still reach the mass capacitance of 284.5 F g^?1 and the areal capacitance of 4030.4 mF cm^?2 at 2 mV s^?1 when the loading is 14.2 mg cm^?2 in 1 M H₂SO₄ electrolyte. In addition, the electrode showed good cycling performance without capacitor degradation after 20,000 cycles at 50 mV s^?1. These results suggest that by using the strategy of building special 3D structure of 2D MXene with 1D unidirectional channels, high performance supercapacitor electrodes with high mass loading can be realized.     

Bifunctional Ti3C2Tx–CNT/PANI composite with excellent electromagnetic shielding and supercapacitive performance

Ti₃C₂T_x exhibits excellent electromagnetic (EM) shielding and electrochemical properties. However, the inherent re-stacking tendency and easy oxidation of Ti₃C₂T_x limit its further application. In this study, a multi-walled carbon nanotube/polyaniline composite (CNT/PANI, denoted as C–P) was introduced into Ti₃C₂T_x nanosheets to obtain a Ti₃C₂T_x–CNT/PANI composite (T@CP). Owing to the integrated effects of Ti₃C₂T_x and C–P, the contribution of absorption was significantly improved, which finally enhanced the EM shielding performance of T@CP. The highest total EM shielding effectiveness (SE_T) was close to 50 dB (49.8 dB), which was substantially higher than that of pure Ti₃C₂T_x (45.3 dB). Moreover, T@CP demonstrated outstanding supercapacitive performance. The specific capacitance of T@CP (2134.5 mF/cm² at 2 mV/s) was considerably higher than that of pure Ti₃C₂T_x (414.3 mF/cm² at 2 mV/s). These findings provide a new route for the development of high-efficiency Ti₃C₂T_x-based bifunctional EM shielding and electrochemical materials.     

Electrochemical storage mechanism of interstratification-assembled Ti3C2Tx MXene/NiCo-LDHs electrode in alkaline,acid and neutral electrolytes

Different kinds of two-dimensional hybrid electrodes have high theoretical capacitance and energy density. However, the origin of the electrochemical storage mechanism still remains elusive in alkaline, acid and neutral electrolytes. Herein, the interstratification-assembled Ti₃C₂T_x MXene/NiCo-LDHs electrodes were successfully prepared and studied in different electrolytes by in-situ Raman spectroscopy. The results show that H₂O molecules in neutral electrolyte combine with –OH at the end of Ti₃C₂T_x MXene during charging, and debonding occurs during discharge. Similarly, this reaction also occurs in the discharge process with NiCo-LDHs and provides smaller pseudocapacitance characteristics. Although this pseudocapacitance reaction also occurs in acidic and alkaline electrolytes, however, the difference is that the hydrogen ions will promote the electrochemical performance of Ti₃C₂T_x MXene and has a certain corrosion consumption effect on NiCo-LDHs, but generally improve the electrochemical performance of Ti₃C₂T_x MXene/NiCo-LDHs. Interestingly, the OH^? in alkaline electrolyte can promote the electrochemical performance of NiCo-LDHs, and produce a new electrochemical reaction with –F between the layers of Ti₃C₂T_x MXene, which greatly improves the overall electrochemical performance of this hybrid electrodes. As a result, Ti₃C₂T_x MXene/NiCo-LDHs electrodes have the best electrochemical performance in alkaline electrolyte with capacitance of 283 F g^?1, energy density of 14.2 Wh kg^?1 and power density of 3007.1 W kg^?1. This work lays a foundation for the preparation of high-performance two-dimensional hybrid electrochemical energy storage devices.     

A novel graphene-like titanium carbide MXene/Au–Ag nanoshuttles bifunctional nanosensor for electrochemical and SERS intelligent analysis of ultra-trace carbendazim coupled with machine learning

A new bifunctional intelligent nanosensing platform based on graphene-like titanium carbide MXene (Ti₂C MXene)/Au–Ag nanoshuttles (NSs) for both electrochemical and surface-enhanced Raman scattering (SERS) intelligent analysis of ultra-trace carbendazim (CBZ) residues in tea and rice coupled with machine learning (ML) was successfully designed. Ti₂C MXene was synthesized by selectively etching Al layers of Ti₂AlC with hydrofluoric acid and high-temperature calcination. Ti₂C MXene/Au–Ag NSs prepared by the ultrasonic dispersion of graphene-like Ti₂C MXene into Au–Ag NSs solution under dark conditions displayed large and rough surface, enhanced conductivity, excellent electrochemical response, prominent Raman enhancement, and high stability. The ML via different algorithms such as artificial neural network, support vector machine, and relevance vector machine (RVM) for the intelligent analysis of CBZ was contrasted and discussed. RVM displayed more superiority for the electrochemical analysis of CBZ in a wide linear range of 0.006 – 9.8 μM with low limit of detection (LOD) of 0.002 μM and SERS detection of CBZ in the wide linear range of 0.033 – 10 μM with low LOD of 0.01 μM. This will provide a new bifunctional intelligent sensing platform via different ML algorithms for improving accuracy of sensor via mutual verification of two or more methods of detection and a new bifunctional nanosensing platform based on the development of graphene-like nanohybrid for food and agro-products safety.     

Fabrication of PANI@Ti3C2Tx/PVA hydrogel composite as flexible supercapacitor electrode with good electrochemical performance

Developing a new strategy to effectively prevent the restacking of MXene nanosheets will have significant impacts on designing flexible supercapacitor electrodes. Herein, a novel Ti₃C₂T_x/polyvinyl alcohol (PVA) porous sponge with 3D interconnected structures is prepared by sol-gel and freeze-dried methods. This Ti₃C₂T_x/PVA porous sponge is used as the template of in-situ polyaniline (PANI) polymerization, and the fabricated PANI@Ti₃C₂T_x/PVA hydrogel composite is applied as flexible supercapacitors electrodes. 1D conductive polymer chains PVA could increase the interlayer spacing of Ti₃C₂T_x nanosheets, which is beneficial to expose more electrochemical active sites. The supercapacitor based on PANI@Ti₃C₂T_x/PVA hydrogel composite exhibits the coexistence of double-layer capacitance and pseudocapacitance behavior. This supercapacitor shows a maximum areal specific capacitance of 103.8 mF cm^?2 at 2 A m^?2, and it also exhibits a maximum energy density of 9.2 μWh·cm^?2 and an optimum power density of 800 μW cm^?2. The capacitance of this supercapacitor is almost not change under different bending angles. Moreover, 99% capacitance retention is achieved after 10 000 charge/discharge cycles of the supercapacitor. The synergistic effect between PANI and Ti₃C₂T_x/PVA composite may improve the number of reactive sites and provide efficient channels for ion diffusion/electron transport.     

Effect of Ti3C2Tx/Ag MXene fillers on the electrical conductivity of Ag-coated Cu conductive adhesives

In this paper, we prepared ECAs (electrical conductive adhesives) with high electrical conductivity by using Ag-coated copper powder and M − II (Ti₃C₂T_x/Ag powder) as conductive filler. M-Ⅰ (Ti₃C₂T_x) and M-Ⅱ were prepared by acid etching and hydrothermal methods, respectively, and the electrochemical properties of M-Ⅱ and the effects of different contents of M-Ⅱ on the conductivity of ECAs were investigated. M − II has a unique reticular structure which helps to create conductive pathways, and its high specific surface area provides a large number of active sites for charge storage, resulting in increased electrical conductivity. The experimental results show that the best conductivity is achieved when the M-Ⅱ powder content is 1.2%, with a volume resistivity of 4.84 × 10⁻⁶ Ω m, at which time the Ag-coated Cu powder content is 69.8%. It has been shown in many studies that only when Ag-coated copper powder content is 70%–80%, the resistivity reaches about 10⁻⁶ Ω m. This work showed that adding a small amount of M − II and reducing the amount of Ag-coated copper powder could maintain the high electrical conductivity of ECAs.     

Subsize Ti3C2Tx derived from molten-salt synthesized Ti3AlC2 for enhanced capacitive deionization

Ti₃C₂T_x is a promising intercalation-type electrode material for capacitive deionization (CDI). However, Ti₃C₂T_x, obtained from traditional synthesized Ti₃AlC₂, is with large particle size and undersized interlayer space, which can easily lead to the longer ion diffusion path, fewer adsorption sites, and higher ion diffusion barrier in CDI process. In this work, subsize and Na⁺ intercalated Ti₃C₂T_x (Na⁺-Ti₃C₂T_x-MS) was prepared by HF etching KCl-assisted molten-salt synthesized Ti₃AlC₂ and following NaOH treatment. The Na⁺-Ti₃C₂T_x-MS achieves a high electrosorption capacity of 14.8 mg/g and a high charge efficiency of 0.81 at the applied voltage of 1.2 V in 100 mg/L NaCl solution. Besides, the stable desalination performance of Na⁺-Ti₃C₂T_x-MS has been confirmed. The superior performance of Na⁺-Ti₃C₂T_x-MS can be attributed to the subsize particle and larger interlayer space. Both two factors can effectively increase ions adsorption sites, shorten diffusion path lengths, and reduce diffusion barriers in the CDI process.     

Improvement of gas sensing property for two-dimensional Ti3C2Tx treated with oxygen plasma by microwave energy excitation

Two-dimensional layered Ti₃C₂T_x MXene was prepared through hydrothermal etching method with LiF and hydrochloric (HCl) acid. Ti₃C₂T_x was further treated with oxygen plasma activated by microwave energy to obtain the activated Ti₃C₂T_x at different temperatures ranging from 350 °C to 550 °C. The gas-sensing properties of raw Ti₃C₂T_x and Ti₃C₂T_x activated with oxygen microwave plasma were tested toward different volatile organic compounds gases. The results indicated that Ti₃C₂T_x activated at 500 °C exhibited excellent gas-sensing properties at room temperature (25 °C) to 100 ppm ethanol with a value of 22.47, which is attributed to the enhancement of the amount of oxygen functional groups and defects on the MXene Ti₃C₂T_x film, and in turn to lead to more oxygen molecules adsorption and desorption reaction in the active defect sites. The enhancement of ethanol-sensing performance demonstrated that the activated Ti₃C₂T_x possess great potential in gas sensing.     

Manganese dioxide nanosheets decorated on MXene (Ti3C2Tx) with enhanced performance for asymmetric supercapacitors

The incorporation of nanosized pseudocapacitive materials and structure design are general strategies to enhance the electrochemical performance of MXene-based materials. Herein, the decoration of manganese dioxide (MnO₂) nanosheets on MXene (Ti₃C₂T_x) surfaces was prepared by a facile liquid phase coprecipitation method. Ti₃C₂T_x is initially modified by polydopamine (PDA) coating to ensure the homogeneous distribution of MnO₂ nanosheets and tight and close connections between MnO₂ and the Ti₃C₂T_x backbone. Due to the obtained three-dimensional (3D) nanostructure, facilitating electron transport within the electrode and promoting electrolyte ion accessibility, the δ-MnO₂@Ti₃C₂T_x-0.06 electrode yields superior electrochemical performances, such as a rather large areal capacity of 1233.1 mF cm^?2 and high specific capacitance of 337.6 F g^?1 at 2 mV s^?1, as well as high cyclic stability for 10000 cycles. Furthermore, δ-MnO₂@Ti₃C₂T_x-0.06 composites are employed as positive electrodes, and activated carbon (AC) materials act as negative electrodes with an aqueous electrolyte of 1 M Na₂SO₄ to assemble asymmetric supercapacitors. The prototype device is reversible at cell voltages from 0 to 1.8 V, and manifests a maximum energy density of 31.4 Wh kg^?1 and a maximum power density of 2700 W kg^?1. These encouraging results show enormous possibilities for energy storage applications.     

Ultrathin Ti3C2Tx nanosheets modified separators for lithium–sulphur batteries

Currently, among the various emerging energy storage systems, the lithium–sulphur (Li-S) battery is expected to be one of the next-generation lithium secondary batteries with high efficiency. However, the practical application of Li-S batteries still faces many obstacles. To solve the shuttle effect of lithium polysulphides, ultrathin Ti₃C₂T_x nanosheets were prepared through the in-situ acid etching method and applied to separator modification to suppress the shuttle effect of lithium polysulphides. Ultrathin Ti₃C₂T_x nanosheets with enlarged interlayer spacing accelerated the migration of Li⁺. The abundant termination groups on the surface of Ti₃C₂T_x played the role of the lithium polysulphide capture centre. When the mass loading of separator modification materials was set as 0.025 mg cm⁻², the as-prepared battery exhibited a reversible specific capacity as high as 780 mAh g⁻¹ after 200 cycles at 0.2 C, and the single-cycle capacity decay rate was only 0.09%.     

Core@shell and sandwich-like Ti3C2Tx@Ni particles with enhanced electromagnetic interference shielding performance

Novel and highly effective electromagnetic interference (EMI) shielding materials are desirable to attenuate unwanted electromagnetic radiation or interference produced by electrical communication devices. Here, functional Ti₃C₂T_x@Ni particles with a core@shell and sandwich like structure were fabricated using the facile electroless plating technique. The core@shell structured Ti₃C₂T_x@Ni consists of a Ti₃C₂T_x core and a Ni shell. In the core, thin Ni layers are sandwiched in between Ti₃C₂T_x flakes. EMI shielding effectiveness (SE) values of Ti₃C₂T_x@Ni/wax composites increased with increasing Ti₃C₂T_x@Ni content. The average EMI SE value of 60 wt% Ti₃C₂T_x@Ni/wax composite was 43.12 dB, increased by 73% as compared with 24.93 dB for the same content of pristine Ti₃C₂T_x in wax in the frequency range 2–18 GHz. An average EMI SE of 74.14 dB was achieved in the 80 wt% Ti₃C₂T_x@Ni/wax. The enhanced EMI shielding performance should be ascribed to the synergic effect of the absorption loss of the Ti₃C₂T_x core and the magnetic loss of the Ni shell and the inner Ni layers.     

Fabrication of Ti3C2Tx/In2O3 nanocomposites for enhanced ammonia sensing at room temperature

Ti₃C₂T_x, as a novel two-dimensional material, displays promising prospects in NH₃ detection at room temperature. However, the NH₃ detection limit of pristine Ti₃C₂T_x is still a major research concern. Therefore, it is important to explore new Ti₃C₂T_x-based nanocomposites for better NH₃-sensing performance. In the present experiment, Ti₃C₂T_x/In₂O₃ nanocomposites were successfully synthesized by ultrasonication and characterized by XRD, FESEM, TEM, XPS, and BET. The optimal Ti₃C₂T_x/In₂O₃-based sensor had a high response of 63.8% (30.4 times higher than that of pristine Ti₃C₂T_x) to 30 ppm NH₃ at room temperature. In addition, the optimal Ti₃C₂T_x/In₂O₃-based sensor had stable repeatability, excellent selectivity, and long-term stability, while exhibiting excellent potential for NH₃ detection at room temperature.     

Ti3C2Tx-foam as free-standing electrode for supercapacitor with improved electrochemical performance

To prevent restacking of the Ti₃C₂T_x layers, the Ti₃C₂T_x-foam has been successfully synthesized through thermal treatment of Ti₃C₂T_x-film with the hydrazine monohydrate. The interconnected porous structure of Ti₃C₂T_x-foam could effectively reduce the restacking of the Ti₃C₂T_x sheets and shorten the diffusion path of ions and accelerate the intercalation/de-intercalation of ions. The Ti₃C₂T_x-foam-80 used as free-standing electrode achieves a high areal capacitance of 271.2 mF/cm² (122.7?F/g) at a scan rate of 5?mV/s in 1?M KOH electrolyte. It also exhibited a high capability rate of 65.5% from 5?mV/s to 100?mV/s and good cycle life with 88.7% retention of its initial after 10,000 cycles at a scan rate of 50?mV/s.     

Multilayer Ti3C2Tx: From microwave absorption to electromagnetic interference shielding

Ti₃C₂T_x MXene has attracted extensive attention in the field of electromagnetic (EM) protection over recent years. Multilayer Ti₃C₂T_x (M-Ti₃C₂T_x), as an intermediate product of MXene ultra-thin structure, has potential advantages in the field of EM protection. Herein, the M-Ti₃C₂T_x was obtained by HCl/LiF etching Ti₃AlC₂. The microwave absorption (MA) and electromagnetic interference (EMI) shielding performance of Ti₃AlC₂ and M-Ti₃C₂T_x were compared. The mechanism research of MA and EMI shielding indicates that the construction of local conductive network plays a leading role in the EM wave attenuation. The sample with 30% M-Ti₃C₂T_x display RL_min of ?50.26 dB, and corresponding bandwidth of 4.64 GHz at the thickness of 1.7 mm. Especially, the metastructure based on the EM parameters of M-Ti₃C₂T_x/wax exhibits ultra-wide bandwidth (15.54 GHz). Our research will provide a basis for the design of MXene-based EM protection performance.     

Broadband electromagnetic absorption of Ti3C2Tx MXene/WS2 composite via constructing two-dimensional heterostructure

Ti₃C₂T_x MXene, an emerging two-dimensional (2D) ceramic material, has rich interfaces and strong conductive networks. Herein, we have successfully built a heterostructure between Ti₃C₂T_x MXene and WS₂ to improve electromagnetic absorption performance. X-ray diffraction and X-ray photoelectron spectroscopy were used to determine the successful synthesis of Ti₃C₂T_x/WS₂ composite. Field emission scanning electron microscopy and transmission electron microscopy images show that WS₂ nanosheets are evenly dispersed on the accordion-like Ti₃C₂T_x MXene. Importantly, Ti₃C₂T_x MXene/WS₂ composite has sufficiently high dielectric loss and impedance matching due to self-adjusting conductivity and 2D heterostructure interfaces. As a result, the Ti₃C₂T_x/WS₂ composite has a minimum reflection loss (RL_min) of −61.06 dB at 13.28 GHz. Besides, it has a broad effective absorption bandwidth (EAB) of 6.5 GHz, with EAB >5.0 GHz covering a wide range of thickness. Such impressive results may provide experience for the application of Ti₃C₂T_x ceramics and 2D materials.     

Multi-phase heterostructures of flower-like Ni(NiO) decorated on two-dimensional Ti3C2Tx/TiO2 for high-performance microwave absorption properties

Three-dimensional flower-like Ni(NiO) decorated on two-dimensional Ti₃C₂T_x/TiO₂ composites were successfully synthesized by an in situ solvothermal reaction, and the electromagnetic (EM) wave absorption performance of the hybrids were explored at 2.00–18.00 GHz. The as-prepared Ni(NiO)/Ti₃C₂T_x/TiO₂ composites include flower-like Ni(NiO) with uniform distribution on the surface of Ti₃C₂T_x MXenes and part of them get into the space between interlayers. The Ni(NiO)/Ti₃C₂T_x/TiO₂ composites exhibit a maximum reflection loss (RL) value of ?41.74 dB at 14.96 GHz with the absorber thickness of merely 1.3 mm and the effective absorption bandwidth (EAB) reaches 3.20 GHz. The outstanding electromagnetic wave absorbing performance can be attributed to the dielectric loss of Ti₃C₂T_x MXenes and multi-phase heterostructures, the magnetic loss of Ni(NiO) and their synergistic loss mechanism. Moreover, the zigzag path formed by flower-like Ni(NiO) also has a great consumption effect on electromagnetic waves by incurring the eddy current under the affect of alternating EM waves. The laminated structure of Ti₃C₂T_x MXenes also dissipates microwaves by offering the space for multiple reflections and scattering. This paper furnished a novel modus for synthesizing original EM wave absorption materials and making the balance among thickness, broad bandwidth, oxidation resistance and light weight, which makes Ni(NiO)/Ti₃C₂T_x/TiO₂ composites a hopeful material for microwave absorption (MA).     

2D/2D SnO2 nanosheets/Ti3C2Tx MXene nanocomposites for detection of triethylamine at low temperature

Highly active two-dimensional (2D) nanocomposites have been widely concerned in the field of gas sensors because of their unique advantages and synergistic effects. 2D/2D SnO₂ nanosheets/Ti₃C₂T_x MXene nanocomposites were synthesized by using layered Ti₃C₂T_x MXene and uniform SnO₂ nanosheets by hydrothermal method. Characterization results show that the SnO₂ nanosheets are well dispersed and vertically anchored on the layered Ti₃C₂T_x MXene surface, forming heterogeneous interfaces. Based on the gas-adsorption capabilities and synergistic effects of electronic properties, SnO₂ nanosheets/Ti₃C₂T_x MXene nanocomposites show high triethylamine (TEA) gas-sensing performance at low temperature (140 °C). The sensor responses of the nanocomposites and pure SnO₂ nanosheets to 50 ppm of TEA are 33.9 and 3.4, respectively. An enhancement mechanism for SnO₂ nanosheets/Ti₃C₂T_x MXene nanocomposites is proposed for highly sensitive and selective detection of TEA at low temperature. The combination strategy of two-dimensional metal oxide semiconductor and multilayer MXene provides a new way for the development of cryogenic gas sensors in the future.